Crustacea.— The structure of the shell in Crustacea has been hitherto examined only in the Decapod order ; and that of the common crab (Platyeareinus pagurus) alone has been subjected to a minute investigation. It is in the Decapod order that the shell attains its most perfect development, and contains the largest proportion of mineral matter : the special respiratory apparatus in this order being so elaborate as to render unnecessary any participation of the general tegumentary surface in the function of respiration. (See vol. i. p. 752.) The shell of the Decapod Crustacea con sists of three layers ; — namely, 1. a horny epidermic membrane covering the exterior ; 2. a cellular or pigmentary stratum ; and 3. a calcareous or tubular substance. The horny epidermic membrane is easily detached from the subjacent layers, after the shell has been immersed for a time in dilute acid ; it is thin but tenacious, presenting no trace of structure, though it may exhibit markings on the under surface, derived from its contact with the cel lular layer beneath. The pigmentary stratum is very thin in the crab and lobster ; but in some other Decapods it is much thicker. In Sryllurus latus,it is stated by M. Lavalle to be the thickest of the three layers of the shell ; and in the cray-fish and many other species, according to the same observer, it seems made up of a considerable number of layers, its ver tical section being traversed by several ex tremely fine lines, passing in a direction parallel to the surface of the shell and to each other. The number of these is usually from six to fifteen ; but they sometimes amount to as many as thirty, or even sixty, their number not being in relation either to the thickness of the pigmentary layer, nor to the size of the species observed ; but appearing to augment with age. The cellular layer is that in which the colouring matter of the shell is solely con tained ; but it does not always contain pig ment, its structure being precisely the same on the white under-surface of the crab as on the reddest portion of its carapace. When examined with a low magnifying power, it pre sents an areolar aspect; but when a suffi ciently thin section is viewed by transmitted light with a high magnifying power, the cha racter of the net-work, and of the dark spaces it encloses, becomes at once apparent. It is possessing considerable firmness, and closely resembling that vvhich is left after the decal cification of dentine. When a thin section of it is made parallel to its surface, and sub jected to a high magnifying power, it is seen to be composed of an apparently homogeneous substance, studded with minute points, each surrounded by a clear space, which correspond with those seen in a section of dentine cut at right angles to the course of its tubuli, and which would seem to possess the satne essen tial character with them. A thin section of the shell taken in the opposite direction (i. e. from surface to surface) leaves no doubt, when examined with a sufficient magnifying power, of the nature of these markings ; for they are then clearly seen to be the orifices of tubuli, which pass with great regularity from one surface of the shell to the other, lying nearly parallel to each other, and having their usually straight course interrupted at tolerably regular intervals by minute sinuosi then obvious that the nearly colourless poly gonal reticulations are the thickened walls of cells, each of them being divided by a distinct line, which marks the junction of the conti guous boundaries ; whilst the tlark spaces or areolm are the cavities of the cells, filled with colouring matter, or with some other semi opaque substance. This cellular layer is not uniformly disposed over the entire surface of the crab-shell ; for the calcareous layer beneath rises up through it in little papillary eleva tions (fig. 425.a), to the summit of which the
epidermis adheres. It is from the deficiency or the pigtnentary layer at these points, that the shell derives its minutely speckled appear ance.
The internal layer is that which constitutes by far the thickest part of the shell of the crab, and which must be regarded as its fun damental or essential element, since (according to M. Lavalle) it is never wanting in the Decapod Crustacea, whilst other layers are sometimes deficient. It is in this internal layer, that the calcareous matter is chiefly de posited ; but even after this has been re moved, a ery distinct animal basis is left, ties resembling the " secondary curvatures " described by Prof. Owen in the dentinal tubuli. These sinuosities correspond with bands which are seen to traverse the section, running parallel to the surfaces of the shell ; and they appear, like those of dentine, to indicate the successive stages of calcification of the animal basis. This structure is par ticularly well seen in the black extremities of the claws of the common crab, in which the intertubular substance is quite transparent in a thin section, and of which the hardness and density are as great as in many varieties of dentine ; and as the tubuli are seen, in a transverse section of the claw, to radiate from the central cavity towards the surface, the resemblance to a section of a tooth is alto gether so close, as quite to deceive an ob server unacquainted with the substance he is examining. The same structure exists, how ever, in the remainder of the shell ; but from some difference in its molecular constitution, the intertubular substance has a less dense and tenacious character, and has an opaque chalky aspect, which renders even a very thin section of it impermeable to light, unless it be saturated with Canada balsarn, which then very commonly enters the tubuli, and prevents them from being readily distinguishable. The purpose of the extraordinary density possessed by the extremities of the claws, is evidently to adapt them to the various mechanical uses to which the animal applies them : and it is in teresting to see that this is attained without any variation in the organi structure of the part, but merely by a more intimate union, as it would seem, of the solidifying mineral matter with the organic basis. It does not seem improbable that the phosphate of lime which is known to be present with the carbo nate in the shells of Crustacea, may exist in larger proportion towards the extremities of the claws than in other parts of the shell ; a question w ell worthy of chemical investigation.
The periodical exuviation of the shell does not appear to be common to all Crustacea; for, according to Mr. Couch*, it does not take place in many of the sessile-eyed tribes, whose cases are as dense as those of the pe dunculate orders. It is much to be desired that careful observations should be made on the formation of the new shell in the Crab ; since these would probably throw light on much that still remains obscure in the de velopment of dentine.
I-The author of the forgoing article is de sirous that it should be understood that all the statements contained in it, except such as are expressly made on the authority of others, are the result of his own observations ; the general facts regarding the organic structure of the shells of Mollusca, Echinodermata, and Crustacea, having been determined by him in the year 1842, and embodied in a paper read before the Royal Society, Dec. 22 of that year, of which the first of the memoirs cited is an abridgment ; and the subject having been subsequently worked out by him in de tail, with the aid and encouragement of the British Association, to the reports of which he would refer the reader vvho may desire additional information as to the results of his researches.1